1. Field of the Invention
The present invention relates to a method of controlling the swing motion of a revolving superstructure such as for a shovel loader or a backhoe, capable of controlling the hydraulic pressure for driving a hydraulic motor for slewing the revolving superstructure, and a hydraulic control system for carrying out the same.
2. Description of the Prior Art
FIG. 6 shows a hydraulic backhoe comprising a base carrier 110, a revolving superstructure 120 mounted for swing motion on the base carrier 110, a boom 130 pivotally supported on the revolving superstructure 120, an arm 140 pivotally joined to the extremity of the boom 130, and a bucket 150 pivotally joined to one end of the arm 140. The boom 130, the arm 140 and the bucket 150 are operated respectively by a boom cylinder actuator 131, an arm cylinder actuator 141 and a bucket cylinder actuator 151.
The revolving superstructure 120 is driven for swing motion and is braked to a stop by a slewing mechanism as shown in FIG. 7. Referring to FIG. 7, the revolving frame 121 of the revolving superstructure 120 is supported for swing motion on a swing bearing 160 on a lower frame 111 mounted on the base carrier 110. The outer race 161 of the swing bearing 160 is fixed to the revolving frame 121. The inner race 162 of the swing bearing 160 is provided with an internal gear 163, and a hydraulic motor 170 and a reduction gear 171 are mounted on the revolving frame 121 with a pinion 172 fixedly mounted on the output shaft of the reduction gear 171 in engagement with the internal gear 163. The hydraulic motor 170 is controlled to drive or to brake the revolving superstructure 120 through the reduction gear 171, the pinion 172 and the internal gear 163.
J.P. Provisional Pub. No. 53-21379 discloses a hydraulic circuit as shown in FIG. 8 for a hydraulic crane, capable of regulating driving pressure for driving the revolving superstructure and braking pressure for braking the revolving superstructure according to the operated angle of a swing motion control lever 200. The swing motion control lever 200 is shifted in the normal direction indicated by an arrow head A to a driving position, so that a pilot valve 210 supplies a pilot fluid of a pilot pressure in a direction B to set a swing mode selector valve 220 in a driving position, i.e., a position shown on the left side in FIG. 8. Then, a working fluid at a line pressure discharged from a hydraulic pump 230 is supplied to a hydraulic motor 240 in a direction D to drive the hydraulic motor 240 for rotation in a direction E. At the: same time, the pilot pressure is applied to the relief pressure control unit 251 of a supply pressure relief valve 250 associated with the supply side of the hydraulic motor 240 (corresponds to hydraulic motor 170 of FIG. 7) to control the relief pressure, according to the pilot pressure and thus the motor driving pressure for driving the hydraulic motor 240 is controlled. When the swing motion control lever 200 is returned to a neutral position, the swing mode selector valve 220 is set in a neutral position to connect lines 241 and 242 connected to the opposite sides of the hydraulic motor 240 so that the hydraulic motor 240 continues inertial rotation for a so-called neutral idle operation.
The swing motion control lever 200 is shifted from the neutral position in the reverse direction indicated by an arrow head A' to a braking position for positive braking, whereby the pilot fluid is supplied in a direction B' to set the swing mode selector valve 220 in a braking position, i.e., a position on the right side in FIG. 8, so that the working fluid at the line pressure is supplied in a direction D', i.e., a direction reverse to the direction of flow of the working fluid discharged from the hydraulic motor 240. Consequently, the pressure in the line 242 on the discharge side of the hydraulic motor 240 increases to brake the hydraulic motor 240. At the same time, the pilot pressure is applied to the relief pressure control unit of a discharge pressure relief valve 260 associated with the discharge side of the hydraulic motor 240 to control the relief pressure of the discharge pressure relief valve 260 according to the reverse operated angle of the swing motion control lever 200 and thus the pressure for braking the hydraulic motor 240 is controlled.
It is known from the examination of the engagement of the power transmitting members of this slewing mechanism including the internal gear 163 of the swing bearing 160, the pinion 172, and the power transmitting members of the reduction gear 171 that the power acting on the power transmitting members during a braking operation is greater than that acting on the power transmitting members during a driving operation due to differences between the braking operation and the driving operation in power transmitting direction even if the effective pressure difference in the hydraulic motor 240 is the same. Consequently, the working surfaces of the power transmitting members acting during the braking operation (hereinafter referred to as "braking surfaces") are abraded more rapidly than the working surfaces of the same acting during the driving operation (hereinafter referred to as "driving surfaces"), and hence the braking surfaces are required to have a high fatigue strength. Thus the life of the slewing mechanism is dependent on the life of the braking surfaces of the power transmitting members. Accordingly, it is desirable to design and to operate the internal gear 163, the pinion 172 and the reduction gear 171 so that the life of the driving surfaces thereof and that of the braking surfaces thereof are the same. Since the critical tilt angle, namely, a limit tilt angle of the revolving superstructure below which the revolving superstructure is able to swing, is one of the criteria for evaluating the swinging performance of the hydraulic backhoe, it is desirable to increase the driving pressure to the highest possible level to increase the torque for slewing the revolving superstructure.
When the foregoing hydraulic circuit shown in FIG. 8 is applied to the slewing mechanism, the set point for the driving operation of the supply pressure relief valve 250 associated with the supply side of the hydraulic motor 240 and the set point for the braking operation of the discharge pressure relief valve 260 associated with the discharge side of the hydraulic motor 240 are controlled similarly according to the operated angle of the swing motion control lever 200. Accordingly the maximum braking pressure coincides with the maximum driving pressure and hence an excessive force acts on the braking surfaces of the slewing mechanism in braking the revolving superstructure to shorten the life of the component members of the slewing mechanism remarkably.